1. Field of the Invention
This invention relates to a method for evaluating a bonded wafer formed by directly bonding a wafer for active layer to a wafer for support substrate without using an insulating film and thinning the wafer for active layer.
2. Description of the Related Art
Recently, semiconductor substrates having a SOI structure by forming a silicon layer or so-called active layer on an oxide film are adaptable for the speeding-up of devices and low in the power consumption and excellent in the pressure resistance, environmental resistance and the like, so that they are used as a wafer for high-performance LSI in electron devices. Particularly, it is required to produce bonded wafers having a higher quality accompanied with a high integration of the semiconductor device, and hence it is increasingly demanded to produce a bonded wafer by thinning a thickness of a buried oxide film (e.g. thickness of about 20 nm) as compared with the conventional SOI wafer.
Also, as a wafer used for a low power consumption device in subsequent generations, there is mentioned a novel bonded wafer formed by removing an oxide film from surfaces of a wafer for active layer and a wafer for support substrate with HF liquid or the like and directly bonding these wafers without using the oxide film and then thinning the wafer for active layer as disclosed, for example, in JP-A-H05-211128. Such a bonded wafer is noticed as a beneficial wafer in a point of the simplification of the production process and improvement of performances in a mixed crystal orientation substrate.
However, the bonded wafer formed by directly bonding without the insulating film as mentioned above has a problem that at the production step of the bonded wafer, native oxide or water on the surface of the wafer is locally aggregated to form island-shaped oxide, which partly remains in a bonded interface. The presence of such an oxide causes the deterioration of the device characteristics and forms a nucleus causing defects at the production step of the device to lower the chip yield or the like.
Therefore, when the bonded wafer is produced by direct bonding without the insulating film, it is required to suppress the formation of the oxide in the bonded interface as long as possible, but also it is an important subject matter to develop a method for detecting island-shaped oxides, which are apt to be generated in the bonded interface of the bonded wafer.